Crossing the line, a tradition that dates back to ancient times when ships first sailed the world. The "line" is the equator. And If you've crossed it, and gone through the initiation, then you are a Trusty Shellback. If you haven't, then you are a lowly, slimy pollywog.
Exactly how a pollywog becomes a Shellback differs from ship to ship. It really is at the discretion of the Shellbacks as to what the pollywog must go through for most of the day.